The link from a mobile station to a basestation in 3G (third generation) CDMA (code division multiple access) systems is a multiple access radio channel that uses CDMA as the access scheme. In this scheme many users share the same radio channel and each user is uniquely identified by a spreading code.
In 3G CDMA, the mobile transmitter in the mobile station is used by the end user to access the network. It may provide voice only, data only, simultaneous voice and data, and location services for many different environments including indoor, low mobility, full mobility, and fixed wireless. To support the various end user services, the mobile transmitter divides its channel into multiple subchannels. One of these subchannels is used to manage the air interface resource and aid in signal detection. This is the pilot subchannel. The remaining three subchannels are used to carry user data and to implement the air interface protocol.
The pilot subchannel carries pilot channel bits which have the characteristic that they are known at the receiver, and also carries PC (power control) bits which are easily detectable at the receiver, while the remaining channels carry information which is unknown at the receiver.
It is a well known fact that due to the nature of CDMA, the signals transmitted by one user will occupy the same bandwidth as signals transmitted by other users and in fact all the signals of other users may be modelled as a form of interference.
It is sometimes a requirement to determine the location of a user in a cellular network. Furthermore, the FCC has mandated that emergency calls (911 calls) made from cellular telephones shall be physically locatable to within 125 metres by the year 2001. This mandate was created so that emergency crews (fire, police, etc.) would be able to locate the caller in an emergency situation.
Locating a user in a cellular network is basically a triangulation process. The user's signal is received by several fixed base stations and the difference in the time of arrival of the signal at the base stations is used to estimate the user's position. For this system to work, the base stations must be in physically different locations; the physical location of the base stations must be known; and at least three base stations must be able to detect the user's signal. The first two criteria are usually satisfied by most cellular networks. The final criteria poses a problem in power controlled CDMA networks.
In a power controlled CDMA network (i.e., the 3G Cellular Network), a user's transmitted power is controlled by the base station with which it is communicating. The control algorithm is designed to minimize the amount of power transmitted by the mobile such that it still meets the minimum voice and data quality criteria. This is done in a CDMA network to minimize the amount of interference that each user adds to the other users also communicating with the base station. In the position location context, this has the unfortunate side effect of reducing the user's transmitted power to the point where only one base station can detect the signal, hence making position location impossible.
One way to solve the low power problem is for the mobile to increase its power when it needs to be located. This is a realistic solution since only emergency calls are located and there should only be a small percentage of emergency calls on a system at any one time. Unfortunately, increasing a mobile's power during an emergency call increases the amount of the interference that the other users see and thus degrades the quality of their signals.
Any substantial reduction in interference in a CDMA system is very important because this results in an effective increase in the system signal to noise ratio, and thereby allows a system to be deployed with fewer base stations. One of the most significant costs in deploying a CDMA network is simply that of obtaining rights to install base stations the desired locations. Any decrease in the number of base stations required, even at the expense of a slightly increased cost per basestation, would be desirable.
Many interference cancellation schemes have been proposed for eliminating the entire contribution of all other users. Such schemes are very complex because they require some sort of estimate of the received signals of the other users, and in many cases this estimate is very difficult to produce due to the user's weak signal strength.